1. Field of the Invention
The present invention relates to a telephone system providing a voice communication service by the use of an Internet protocol (IP) telephone terminal to make communication using the IP.
2. Description of the Related Art
In recent years, a voice communication system, which is called a voice over Internet protocol (VoIP) and uses an IP network, has begun to be widely used. Such a system of this type is mainly used for extension communication in an office, etc. Utilizing the VoIP for the extension conversation enables accomplishing a cost reduction due to the use of the IP network and multi-functionality such as cooperation with a personal computer. Further, an IP telephone terminal (Wi-Fi phone) which utilizes not only a wired network but also a wireless LAN to mount a session initiation protocol (SIP) thereon has been shipped to the market.
On the other hand, also in a public switched telephone network that is an outside line (external line), not only conventional data communication but also a service form to achieve voice communication on the IP network has increased. It goes same for a mobile phone terminal using a CDMA/GSM system, etc. Nowadays, a dual terminal, having both functions of a mobile phone in the CDMA/GSM system and a wireless IP telephone set, has been sold.
However, to use such a type of terminal, an existing technique enables only the communication through the same carrier. That is, the terminals, using the identical carriers, for example, mobile phones, simple-type mobile phones (personal handy phones [PHSs], etc.), and IP phone terminals can mutually make a telephone call usually. Therefore, to establish the communication between the external phone terminal and the IP phone terminal, expansion of functions of the voice communication system becomes necessary.
Jpn. Pat. Appln KOKAI Publication No. 2004-32319 discloses a proposal to decrease modification on a side of a mobile phone line network required so as to utilize the mobile phone line network via the IP network. This reference discloses a technique in which a proxy terminal on a side of a public switched telephone line network operates the data according to the protocol in the line network.
Another reference, Jpn. Pat. Appln. KOKAI publication No. 2004-297591, discloses an apparatus and a method for seamlessly switching both line switching voice communication and VoIP voice communication. However, the content described in this reference only shows a mobile communication terminal device, and it does not describe about a system for proving the communication service itself between the IP phone terminal and the line telephone terminal.
The existing technique requires to manage information on subscriber's numbers and IP addresses on the side of the line network and to modify the side thereof so as to enable both line switched voice communication and the VoIP voice communication. The line network having a physical interface such as a signal line, the scale of the modification frequently becomes large and difficult. A specification change such as additions of control items frequently occurs after the modification. This modification is difficult similarly.
On the existing public switched telephone line network, for contracting and charging by a carrier, such processing as terminal certification (authentication) to ensure whether or not a user of a terminal is a true subscriber and measurement of duration of call for each subscriber, have been performed. Terminals not to be charged, such as IP phone terminals for internal communication optionally set by an office and a computer with IP communication software installed therein, are connected to the IP network. Therefore, it is hard for contracting and charging to apply the existing technique to the IP telephone terminal.
An object of the present invention is to provide a telephone system enabling the communication between the IP phone terminal and the outside line phone terminal without having to implement large-scale modification.